GtkTextBuffer::create_mark

Creates a mark with the name specified by name
at the position specified by location.
If name is NULL, the mark is anonymous;
otherwise, the mark can be retrieved by name using the
get_mark() method.
If a mark has left_gravity, and text
is inserted at the mark's current location, the mark will
be moved to the left of the newly-inserted text.

This method emits the mark_set
signal as notification of the mark's initial placement.